The terms ‘gem’, ‘gemstone’ and ‘stone’ are used synonymously with the usual meaning referring to minerals such as diamonds, sapphires, rubies, emeralds and so on. Still, in particular cases, without compromising generality, diamonds will be used The terms ‘inclusion’, ‘flaw’ and ‘defect’ are used synonymously indicating an individual discernable visual irregularity inside the gem.
The term ‘dividing plane’ relates to planes through which a stone is sawed, cleaved or cut by any method into separate parts.
Gemstones, as an ornamental object rather than an industrial one, are valued by their appearance. In gemology, the quality of a gem such as a diamond, is typically determined by the “4C's”, Clarity (internal perfection of the stone), Color (colorless being the more expensive), Cut (consisting of shape, proportions, symmetry, and polish), and Carat (weight).
As for clarity, it is important to identify the location and size of flaws inside a rough stone in order to determine the preferred dividing planes that would yield the greatest value from a given stone. Likewise it is important to identify flaws in a polished stone in order to determine its value.
Everyday practice is a visual examination of the stone by experts who try to assess the location and size of the flaws using their experience and following industry rules. Still, it is a human subjective judgment that depends on a particular person's skill and experience and may vary between different individuals and circumstances. Moreover, when a parcel of gemstones is to be evaluated, it could take a long time to assess each stone, so that the parcel value is deduced upon the examination of representative stones only.
To overcome the manual inconsistency and the labor involved, optical methods and devices have been proposed for the detection of flaws in stones. However, the high refractive index of gems, especially diamonds, causes large refractions of incoming and outgoing light and total internal reflections resulting in multiple deflected images of the flaws.
U.S. Pat. No. 4,259,011 describes how to identify the presence of inclusions but not their location. European patent 1,211,503, presents a possible solution for the locating of inclusions in a transparent and at least partially polished diamond by imaging the diamond twice and analyzing the images by computer so as to localize an inclusion with respect to the outer surface of the diamond. Although this patent makes reference to a refractive index correction factor to be included in the computer's calculations, it does not provide a solution to multiple images produced by a single inclusion.
U.S. Pat. No. 4,049,350 teaches eliminating the refractions and reflections at the facets of a cut stone by submerging the stone in a solution of similar refraction index. It describes how to locate an inclusion in a two dimensional plane by aiming a narrow laser beam at a preferred angle to a particular facet.
U.S. Pat. No. 4,152,069 also teaches submerging a cut stone in such a solution and how to find the inclusion within a three dimensional volume.
Both latter references do not disclose any information on the medium they used to closely match the refraction index of the gem, this being particularly problematic for diamonds that have a very high refraction index. As far as is known to the present inventors, no such liquid has been suggested in the art.
A paper entitled “The Optical Properties of Liquid Selenium” (E. W. Saker, Proc. Phys. Soc. 1952, pp. 785-787) provides some experimental results, including the refraction index of solid and molten selenium in the near infrared region with respect to temperature and wavelength.
The disclosures of all of the above cited references are incorporated herein by reference.